(a) Industrial Field of the Invention
The present invention relates to a multilayer material having a resistance layer formed on an electrically conductive layer, the multilayer material being used for the production of printed circuit boards, and to a multilayer board having a resistance layer, where the multilayer board is produced by using the multilayer material and is used for the production of printed circuit boards.
More specifically, the present invention relates to a multilayer material having a resistance layer formed on an electrically conductive layer, which can be laminated to an electrically insulating support so firmly as to improve greatly the peel strength between the resistance layer of the multilayer material and the insulating support, and to a multilayer board which has a resistance layer and is used for the production of printed circuit boards. The multilayer board is produced by laminating the multilayer material having a resistance layer with an insulating support and exhibits outstanding peel strength between the resistance layer and the insulating support.
(b) Description of the Related Art
In production of a printed circuit board, resistors have been installed generally by soldering resistor components onto a printed wiring board. This method, however, not only requires troublesome operations, but also can be no longer sufficiently adaptable to the recent trend in this field of making printed circuit boards smaller, lighter, and denser or more compact. Thus, the method of soldering resistance components has been replaced by an improved method wherein a printed wiring board having a printed resistor circuit is produced from a copper-clad laminate having a resistance layer, where the copper clad laminate is produced by laminating a copper foil provided with a resistance layer onto an electrically insulating support base, for example, an epoxy resin impregnated glass base, by thermo-compression, with the resistance layer interposed between the copper foil and the insulating support. In this method, an intended printed wiring circuit containing printed resistor components is formed on the electrically insulating support, in the same way as in the formation of the wiring patterns of general printed wiring boards, by patterning the copper foil and the resistance layer employing a subtractive process and a selective etching technique. In forming the printed wiring circuit containing printed resistor components, the copper foil layer and the resistance layer are etched respectively to remove their respective unwanted portions, by using at least two etchants and repeating formation of a resist coating, etching, and removal of the resist coating. Accordingly, in this method, the interface between the resistance layer and the electrically insulating support is apt to be eroded from the sides of the laminate by the erosive action of the acidic or alkaline environment brought by the etchants and the resist removing liquids used, thereby resulting in a decrease in the peel strength (chemical resistance) between the resistance layer and the insulating support. It is, therefore, important for improving reliability of the boards having resistance layers for printed circuit boards to increase the peel strength between the resistance layer and the electrically insulating support including the peel strength under the normal condition and after heat treatment.
There have been some attempts to improve the peel strength between the resistance layer and the insulating support. For example, disclosed in Japanese Patent Application Kokoku Koho (Publication) No. 57-3234 is a method of increasing the peel strength between a resistance layer and an insulating support by using as a material of the resistance layer a resistive material which comprises an electrically plated nickel-phosphorus composition having a content of phosphorus of not more than about 30% by weight and having a content of oxide, hydroxide and/or peroxide of nickel of not less than 50% by weight based on the total of the nickel components.
In addition, disclosed in Japanese Patent Application Kokoku Koho (Publication) No. 55-32001 is a method of increasing the peel strength by using a conductive plate one of the both surfaces of which is roughened by a surface roughening treatment, forming a plated resistive layer on the roughened surface of the conductive plate, and then bonding an insulating support to the surface of the plated resistive layer.
Neither the former method nor the latter method is, however, a method of improving the peel strength by bonding the resistance layer (namely, a metallic or metal-like substance) chemically with the insulating support (namely, an organic substance such as the above-described glass-epoxy resin support). Especially, in the latter method, an increase in mechanical peel strength is attempted by roughening the surface of the conductive plate in anticipation of increasing the surface area of the resistive plated layer adhering to the insulating support. However, the method is encountered with a problem that in case a thick resistive plated layer is formed, the surface area of the plated resistive layer is practically reduced because the roughened surface of the conductive plate is leveled by the thick resistive layer plated thereon, and improvement of the peel strength becomes difficult.
As mentioned above, the conventional methods have failed to provide a multilayer board for the manufacture of printed circuit boards which has a resistance layer therein and has a high peel strength so as to be sufficiently reliable in practical use.